We have furnished, during this fiscal year, further proofs that rapid structural changes in the nerve fibers and cells lie at the base of the process of nerve excitation. To this end we have examined excitable tissues consisting of uniform cellular elements that can be excited in a highly synchronous manner, namely fish electric organs. By stimulating the nerve fibers entering the electric organs of electric eels, catfish and rays, we were able to record distinct mechanical and thermal manifestations of structural changes in the excitable elements in the organs. In fact, the magnitude of swelling and temperature rise which we have observed during excitation of these organs are 10 to 100 times as large as those recorded previously from various nerve fibers and cells. Particularly significant is our discovery of large mechanical changes associated with synaptic activity in these organs. We found that the well-known electric sign of synaptic activation (i.e. synaptic potential) is accompanied by relatively large swelling and temperature rise of the tissue. These early signs of synaptic activity are followed by a slow, but much larger swelling (i.e. an outward movement of the surface) of the organ. These findings offer a strong support to our view that the structural changes in excitable tissues are associated with movements of water molecules in the superficial layer of the nerve fibers and cells. Furthermore, these results strongly suggest that the mechanical and thermal changes in the amphibian spinal cord and brain which we have described previously arise from the synapses rather than from the nerve fibers.